The present invention relates to the field of electronic tuning systems for television receivers and the like including provisions to inhibit the tuning system from locking up on spurious signals generated by the system itself under certain operating conditions.
Electronic tuning systems for radio and television receivers have recently been developed which utilize a voltage controlled oscillator, typically of the varactor diode type, the frequency of which is adjusted in response to a control signal to provide the local oscillator frequency appropriate for tuning a selected station or channel. Although these systems are typically less failure-prone than are their mechanical counterparts, portions of the tuning system may generate spurious signals under certain operating conditions which cause the tuning system to lock up. In U.S. Pat. No. 3,971,991 entitled, "Television Tuning System with Varactor Malfunction Detection", issued in the name of A. Tanaka on July 27, 1976 there is described a tuning system in which the local oscillator signal generated by a varactor controlled oscillator has its frequency in response to a ramp voltage until a counter responsive to the local oscillator signal accumulates the appropriate count for a selected channel. Under some varactor operating conditions, the varactor controlled oscillator signal output may be weak or intermittent and as a result the accumulated count may be erroneously low. In order to inhibit the tuning system from becoming locked up at a local oscillator frequency corresponding to an erroneous count, a comparator to determine the actual count and the desired count and a malfunction detector to drive the varactor controlled oscillator in a direction opposite to that indicated by the comparator are provided.
To simplify this structure, it is desirable that lockup inhibiting provisions in electronic tuning systems automatically prevent lockups due to the generation of spurious signals when they are likely to occur, rather than taking corrective action after the lockups have occurred.